Alectai:
Eh, I wouldn't go too far into gimmick stuff, you won't find a lack of players either for a 40K game.
That's a reason to steer into the gimmick stuff not the other way around. Doesn't matter if you turn off a few people with the concept, there are more to take their place. Less true with OW than with d20 games but still mostly true.
Alectai:
Ultimately, I've not much to add to it, as I usually run Specialists anyway in Only War, hence I'm a bit skeptical about doing a Gue'vesa game, that's the kind of thing that's hard to run a good campaign for without flanderizing anyone involved I feel.
Playing a specialist doesn't exempt you from regimental culture. I'll beat people over the head with that side bar until the sun burns out.
Not sure the point you are trying to make about flanderizing characters. Like even in a Gue'Vesa game, it's going to come up that you are in the Tau empire fighting for the Greater Good a lot. But reasonably no more than how often you mention the Emperor and the Imperium in a normal game. All of the normally available war tropes still apply even if your government is slightly less dysfunctional.